The physical identification and characterization of certain maize transposable elements has made a range of biological phenomena accessible to molecular study. We propose two lines of research pertaining to the Ac/Ds controlling element system. First, we have evidence that activation of Ac elements in the cell may be controlled by structural and/or methylation changes. We propose to study the structure of cryptic copies of Ac and Ds elements in the maize genome and to induce activation of Ac to examine the mechanism of activation. Second, we propose to use insertions of Ac and Ds to clone and characterize the R locus of maize.